First, we summarize results of our recent combined experimental and theoretical study [1], addressing analogies between electronic gap features in the infrared response of the ladder compound Sr_14−xCa_xCu₂₄O₄₁ (SCCO) and those of the underdoped high-T_c superconductor YBa₂Cu₃O_6+x, indicating, in conjunction with earlier studies [2], that the electronic ground state of the ladders involves (a) spin (singlet) correlations, (b) hole pairs and (c) a charge density modulation. An important contribution of [1] is that the presence of the latter modulation is not limited to very low and/or specific values of doping (determined by x). The central question is that of the nature of coexistence of (a), (b) and (c).

A hallmark of the ground state is a pseudogap in the a-axis (field along the rungs) conductivity of SCCO [3,1]. A pseudogap of comparable size occurs also in the a-axis conductivity spectra computed using the t-J model [1]. What is the origin of the latter pseudogap? In order to answer this question and to improve our understanding of the complex ground state of SCCO, we have performed a detailed numerical analysis of differences between the excited states underlying the pseudogap and the ground state. Resulting insights will be presented in the second part of our contribution.

This work has been supported by the project Quantum Materials for Applications in Sustainable Technologies, Grant No. CZ.02.01.01/00/22_008/0004572.

[1] P. Adamus, B. Xu, P. Marsik, A. Dubroka, P. Barabasová, H. Růžičková, P. Puphal, E. Pomjakushina, J. L. Tallon, Y. L. Mathis, D. Munzar, and C. Bernhard, Reports on Progress in Physics 86, 044502 (2023).

[2] For a review, see Vuletic et al., Phys. Rep. 428, 169 (2006).

[3] T. Osafune et al., Phys. Rev. Lett. 82, 1313 (1999).